What Is The Role Of Heterospory And Megaspore In The Evolution Of Seed?


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saima jabeen Profile
saima jabeen answered
The major steps associated with the evolution of seed habit are as follows:
The evolution of heterospory.

Primitive vascular land plants produced spores which were all morphologically and structurally alike, a condition called homospore. All groups of land plants up to Pteridophytes (lower vascular plants such as club mosses, horsetails) are homosporous.During the early phase of evolution some plant groups started producing morphologically, structurally and functionally two different types of spores, the smaller ones called microspores and the larger ones known as megaspores.

The microspores produced inside Microsporangia germinated to form male gametophyte or the micro gametophyte, whereas the megaspores germinated to form female gametophyte or mega gametophyte.

Retention and Germination of Megaspore with in the Megasporangium
During the usual reproductive cycle in the heterosporous vascular land plants, the megaspores used to be shed and dispersed soon after their formation in order to germinate into female gametophyte. However in some plants (e.g., Selaginella) the megaspore was not allowed to escape from megasporangium after its formation. Its release from megasporangium was either delayed or the megaspore(s) was/were permanently retained within the megasporangium. Here, within the confines of the megasporangium wall, the megaspore germinated to form egg containing female gametophytes.
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Michael Sholar
Michael Sholar commented
A seed has an embryo (fertilized egg) surrounded by a protective parent tissue layer (among other things). In order for this to exist, the embryo must form (fertilization must happen) while still attached to the parent plant. If homospory were the case then this would not be possible. Either all spores would leave the parent plant or all would remain attached, either way we would not have seeds. In order for seeds to be produced some spores have to remain attached and some need to be mobile. That is heterospory. Heterospory had to predate seeds.
Michael Sholar
Michael Sholar commented
Seeds are composed of an embryo contained within parent tissues (among other things). In order for this to occur, fertilization has to occur while the spore is attached to the parent plant. If homospory was the case then either all spores would leave the parent plant or all would remain attached. Either way we would not have seeds because fertilization would not occur while attached to the (sporophyte) parent. Heterospory describes a situation in which some spores produce pollen that leave the parent plant and some produce eggs that remain attached. This allows a seed to develop at the site of the attached megaspore (egg producer) because it could remain and still be fertilized. Heterospory had to predate seed.

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